Various types of rear-wheal steering angle control devices have been already known, in which a vehicle running stability and a capability of a vehicle small turning circle can be improved by steering rear wheels as well as front wheels in response to the vehicle driving condition. According to this type of rear-wheel steering angle control device, it is a matter to be discussed how to set an optimal target rear-wheel steering angle relative to a front-wheel steering angle corresponding to every vehicle running condition.
For example, according to a column “7.5.3 4WS control for zero skid” at page 196 of a publication (this publication will be hereafter referred to as “publication No. 1”) entitled “Steering and Dynamics” (date of publication of the first edition: Sep. 10, 1996, publishing office: SANKAIDO, Inc., author: KAYABA KOUGYO, Inc.), the following is disclosed. The vehicle slip angle (a deviation angle between a vehicle running direction and a vehicle longitudinal direction) is first recognized to be important to be focused on. The rear-wheel steering angle is controlled for always setting the vehicle slip angle to be zero. More specifically, when the vehicle speed is equal to a predetermined vehicle speed or less than that, the rear-wheel steering angle is controlled in an opposite direction to the front-wheel steering angle in proportion to the front-wheel steering angle, wherein a small turning circle can be ensured. On the other hand, when the vehicle speed is greater than the predetermined vehicle speed, the rear-wheel steering angle is controlled in the same direction as the front-wheel steering angle in proportion to the front-wheel steering angle so as to obtain an ideal vehicle dynamics with the superior driving stability of the vehicle. Hereinafter, this type of control will be referred to as a “steering angle proportional control”.
As described above, when the steering angle proportional control is applied to the publication No. 1, the rear wheels are steered in the opposite direction to the front wheels when the vehicle speed is within a low speed rage as the vehicle leaves. Therefore, the capability of the small turning circle can be improved. However, the vehicle may become in touch with an obstacle at the vehicle side with the swing-out at the vehicle rear end (in particular, the corner portion of the vehicle rear end portion).
Therefore, to overcome the above described problem of the swing-out at the vehicle rear end ensuring the small turning circle, it is preferable that the rear-wheel steering angle is controlled to follow the vehicle rear end to a locus of a vehicle front end. This control is proposed, for example in a column “105 a control means for restraining a swing-out of a 4WS vehicle” of a publication entitled “The proceedings of Japanese Society of Automotive engineering 882, October, 1988” (this publication will be hereafter referred to as “publication No.2”). More specifically, the vehicle front end running locus is memorized at every predetermined sample distance. The rear-wheel steering angle is controlled for approximately corresponding the vehicle rear end running direction to the vehicle front end running direction at the nearest sample point to the then vehicle rear end among the whole sample points.
However, according to the control proposed in the publication 2, when the steering is turned as the vehicle leaves, the rear-wheel steering angle is first controlled in the same direction as the front-wheel steering angle and is then controlled in the opposite direction thereto. In this case, the driver may feel uncomfortable due to an occurrence of an excessive rear-wheel steering. Further, a consumption energy required for steering the rear wheels may be increased due to the occurrence of the excessive rear-wheel steering.
Accordingly, it is still susceptible of certain improvements with respect to providing an improved rear-wheel steering angle control device which can improve the vehicle small turning circle restraining the swing-out at the vehicle rear end under the vehicle low running speed. Therefore, the driver can be relieved from the uncomfortable driving feeling.